1. Field of the Invention
This invention relates to a semiconductor element such as a field effect thin film transistor, and the like, more particularly to a semiconductor element of which the main part is constituted of a polycrystalline silicon thin film semiconductor layer which is of high quality in behavior characteristics, reliability and stability.
2. Description of the Prior Art
Recently, for constitution of a scanning circuit portion of an image reading device for use in image reading such as one-dimensional photosensor made in a continuous length of two-dimensional photosensor of an enlarged area, or for constitution of a driving circuit portion of an image display device utilizing liquid crystal (abbreviated as LC), electrochromic material (abbreviated as EC) or electroluminescence material (abbreviated as EL), it has been proposed to form a field effect thin film transistor by using as the base material a silicon thin film formed on a certain substrate, corresponding in size to the increased area of such portions.
Such a silicon thin film is desired to be polycrystalline rather than amorphous for realization of a large scale image reading device or image display device improved in having a higher speed and achieving a higher level of function. As one of the reasons, in spite of the requirement that the effective carrier mobility .mu.eff of a silicon thin film as base material for formation of a scanning circuit portion of such a high speed, high function reading device or driving circuit portion of image display device should be large, that of the amorphous silicon thin film obtained by the ordinary discharge decomposition method is at most 0.1 cm.sup.2 /V.sec, and, it also suffers from the drawback that the drain current is reduced as DC voltage is applied to the gate, whereby there is a marked change with the lapse of time such as shifting of the threshold voltage of the transistor, resulting in poor in stability.
In contrast, a polycrystalline silicon thin film has an effective carrier mobility .mu.eff by far greater than an amorphous silicon thin film as can be seen from the data practically measured. Theoretically, it is quite probable that there may be prepared a polycrystalline silicon thin film having a further increased value of mobility .mu.eff than presently obtained.
However, under the present situation, a semiconductor element or a semiconductor device comprising as its main part a semiconductor layer of a polycrystalline silicon thin film prepared by various methods of the prior art does not exhibit sufficiently the desired characteristics and reliability. There have been also made so many studies about the semiconductor elements formed from amorphous silicon thin films containing silicon and fluorine. However, thin film field effect transistors in which amorphous silicons containing silicon and fluorine are used as semiconductor layers have small effective mobilities, and also suffer from the drawback of a large change with lapse of time such that the drain current is reduced as a DC voltage is applied on the gate with a shift of the threshold voltage of the transistor, thus being poor in stability.